In an electrical load dimmer, a technique known as zero crossing detection is conventionally employed, wherein the dimmer is synchronized with one or more phases of an input line voltage to enable the dimmer to properly fire a load controlling switch, such as a TRIAC, at specific firing times with respect to the input line phase. More specifically, a zero crossing is detected by detecting a change in voltage polarity of the input line voltage. In other words, zero crossing is detected when the input line voltage changes polarity at the zero volt level, which triggers a signal in the microprocessor that the voltage level has crossed zero volts.
An electrical load dimmer works by “chopping up” an input line voltage so that the line voltage is delivered to an electrical load only during portions of an input line voltage signal. The line voltage that is delivered to the load by control of the electrical dimmer can be regarded as phase controlled input line voltage. In the case of a light source electrical load, an electrical load can include a light source as well as a driver circuit. A driver circuit among other elements can include a rectifier for rectifying portions of the phase controlled line voltage delivered from the dimmer circuit.
In prior art designs, zero-crossings of an input line voltage are detected by detecting a change in the polarity of the voltage across an input line voltage terminal and an output load terminal (that is, in two wire devices without a neutral connection), or across the input line voltage terminal and return neutral or ground wire terminal (in three wire devices with a neutral connection or two wire devices using a ground leakage path).